Not applicable.
This invention relates to radio frequency (RF) antennas and more particularly to a mechanically steerable RF array antenna.
As is known in the art, satellite communication systems include a satellite which includes a satellite transmitter and a satellite receiver through which the satellite transmits signals to and receives signals from other communication platforms. The communication platforms in communication with the satellite are often located on the surface of the earth or, in the case of airborne platforms, some distance above the surface of the earth. Communication platforms with which satellites communicate can be provided, for example, as so-called ground terminals, airborne stations (e.g. airplane or helicopter terminals) or movable ground based stations (sometimes referred to as mobile communication systems). All of these platforms will be referred to herein as ground-based platforms.
To enable the transmission of radio frequency (RF) signals between the satellite and the ground-based platforms, the ground-based platforms utilize a receive antenna which receives signals from the satellite, for example, and couples the received signals to a receiver circuit in the ground-based platform. The ground-based platforms can also include a transmitter coupled to a transmit antenna. The transmitter generates RF signals which are fed to the transmit antenna and subsequently emitted toward the satellite communication system. The transmit and receive antennas used in the ground-based platforms must thus be capable of providing a communication path between the transmitter and receiver of the ground-based platform and the transmitter and receiver of the satellite.
To establish communication between one or more satellites and the ground-based platform, the antenna on the ground-based platform must be capable of scanning the antenna beam to first locate and then follow the satellite. One type of antenna capable of scanning the antenna beam is an electronically steerable phased array (ESA) antenna. One problem with ESA antennas, however, is that they are relatively large and expensive. Thus ESA antennas are not typically appropriate for use with those ground-based platforms which are frequently moved from one location to another.
Furthermore, although ESA anteinas can rapidly change the position of the antenna beam, such antennas still provide only a single antenna beam at any instant in time. Thus, ESA antennas only allow communication with one satellite at a time. Stated differently, ESA antennas only allow sequential communication with satellites.
Sequential operation is used in communication systems having a so-called xe2x80x9cbreak-before-makexe2x80x9d capability. In this type of communication system, a ground-based platform xe2x80x9cbreaksxe2x80x9d communication with a satellite prior to establishing communication with another satellite. Such communication systems can utilize a single beam antenna system (e.g. an ESA antenna) which can acquire each satellite system sequentially.
Some communication systems, however, require a so-called xe2x80x9cmake-before-breakxe2x80x9d capability. In make-before-break communication systems, a ground-based platform does not break communication with a satellite until it has already established communications with another satellite. To communicate with multiple satellites simultaneously, the ground-based platform must have an antenna system which simultaneously provides multiple antenna beams. Since ESA antennas can only provide a single beam, in order to provide two beams, it is necessary for the ground-based plant form to utilize two ESA antennas. Thus, communication systems which utilize ESA antennas and which have a make-before-break capability can be prohibitively expensive.
Some prior art ground-based platforms utilize frequency scanning antennas. In a frequency scanning antenna, the antenna beam position (also referred to as the antenna scan angle) changes as the operating frequency of the antenna changes. Since the position of any single satellite is relatively constant, once a communication path is established between the satellite antenna and the ground-based platform antenna, changing the scan angle of the ground-based platform antenna can result in the loss of the established communication path. Thus, it is generally not desirable for the scan angle to change once a communication path is established.
To prevent the scan angle from changing, frequency scanning antennas must operate over a relatively narrow band of frequencies. Different communications systems, however, operate at different frequencies spread across a relatively wide frequency range (e.g. the K and Ka band frequency ranges). Since frequency scanning antennas only operate over a relatively narrow band of frequencies, such antennas are typically compatible with only a single satellite communication system (i.e. a single system which operates over a relatively narrow band of frequencies). Thus, it is typically necessary to provide a different antenna with each different ground-based platform operating with different satellite communication systems.
It would, therefore, be desirable to provide a reliable antenna which is relatively low cost and compact compared with the cost and size of an ESA antenna. It would be further desirable to provide an antenna which can be used with a ground terminal, in an airborne station such as an airplane or a helicopter, on a mobile ground vehicle such as a HUMV. It would be still further desirable to provide an antenna which operates over a relatively wide frequency range while providing an antenna beam which is steerable over the entire frequency range such that the antenna is compatible with many different satellite communication systems each of which operates at a different frequency in the operating frequency range of the antenna.
In accordance with the present invention, an antenna includes a lower plate assembly for providing a feed signal on a first surface thereof in response to an input signal provided to an antenna port thereof and an upper plate assembly having a feed circuit coupled to a plurality of radiating elements which define a radiating aperture. The upper plate assembly is rotatably disposed on the first surface of the lower plate assembly such that the feed circuit couples energy between the lower plate assembly and the plurality of radiating elements. A position of the feed circuit on the upper plate assembly relative to the lower plate assembly determines a scan angle of the antenna. With this particular arrangement, an antenna capable of scanning its antenna beam by changing the angle between the feed circuit on the upper plate assembly and the lower plate assembly is provided. The lower and upper plate assemblies can be provided from parallel plate waveguides. The waveguides in each of the lower and upper plate assemblies are aligned and the feed circuit in the upper plate assembly can be provided as a line coupler (e.g. a slot) which couples energy between the parallel plate waveguide transmission line and a corporate feed. The angle at which the line coupler intercepts feed signals on the lower plate assembly determines the antenna scan angle in the elevation plane. Thus, changing the angle at which the line coupler intercepts feed signals on the lower plate assembly changes the antenna scan angle in the elevation plane.